For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 825
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-SubTopic 10
-Section 50
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

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- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

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- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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Related party transactions - Narrative (Details)	3 Months Ended		9 Months Ended
Related party transactions - Narrative (Details)	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
BHH [Member]
Related Party Transaction [Line Items]
Ownership percentage	40.00%		40.00%
Consolidated Sales [Member] \| Customer Concentration Risk [Member] \| Affiliated Entity [Member]
Related Party Transaction [Line Items]
Major customer, percentage of revenue, net (percent)	63.00%	75.00%	63.00%	76.00%
Glencore [Member]
Related Party Transaction [Line Items]
Ownership percentage by majority shareholder	47.40%		47.40%
Common Stock [Member] \| Glencore [Member]
Related Party Transaction [Line Items]
Ownership percentage by majority shareholder	42.90%		42.90%